1. Field of the Invention
The present invention relates to a method of producing ferrous sintered alloys with superior abrasion resistance which are used for forming components of a valve operating mechanism in a engine or the like.
2. Description of the Prior Art
There has been proposed to make movable components of an engine which are required to have sufficient abrasion resistance, such as rocker arms, of ferrous sintered alloy which is obtained by sintering a green compact formed through compression molding of ferroalloy powder. One of ferrous sintered alloys proposed previously for use of making the movable components of the engine thereof is disclosed in, for example, the Japanese patent application published before examination under publication No. 59/83704. The ferrous sintered alloy thus proposed previously is obtained by sintering a green compact of ferrous eutectic alloy powder which contains carbon, boron, molybdenum, phosphorus and other similar elements, and includes in its matrix structure boron carbide, molybdenum carbide, phosphorus carbide and other simple carbides, together with compound carbides, so as to have improved abrasion resistance inherent therein.
The ferrous eutectic alloy powder which contains carbon and phosphorus and is used as raw material of the ferrous sintered alloy is produced generally through processes of melting metals of several kinds including iron and mixed with one another at a mutual weight ratio predetermined in accordance with an expected characteristic of the ferrous sintered alloy, solidifying the melted metals to obtain an alloy ingot, and grinding the alloy ingot with use of, for example, a suitable stamp mill into powder. In such processes for producing the ferrous eutectic alloy powder, it is usual that the alloy ingot has portions unhomogeneous in internal structure of solidified metals, in other words, the alloy ingot is attended with segregation resulting from a difference in solute concentration between a solid phase portion and a liquid phase portion both appearing to form a boundary therebetween at an initial step of melting of the metals.
When the green compact which is formed through compression molding of the ferrous eutectic alloy powder thus produced by grinding the alloy ingot attended with segregation into powder is sintered, low melting point portions of the ferrous eutectic alloy powder forming the compact and having internal structures affected with the segregation melt antecedently to other portions to become liquid phase components and the crystal grain boundary of the alloy is filled with the liquid phase components having arisen thus. Accordingly, the crystal grains of the alloy are drawn to one another with the surface tension of the liquid phase components and thereby coupled with one another under the condition in which pores and other undesirable spaces are restrained from arising. Consequently, the ferrous sintered alloy with improved abrasion resistance is obtained.
However, in the case where such a method including processes of melting the metals including iron and mixed with one another, solidifying the melted metals to obtain the alloy ingot, and grinding the alloy ingot into powder as described above is adopted to produce the ferrous eutectic alloy powder used as raw material of the ferrous sintered alloy, there is a disadvantage that the production cost of the ferrous eutectic alloy powder is increased.
Meanwhile, there has been also proposed an atomization method through which a ferrous eutectic alloy powder containing carbon and phosphorus is obtained directly from melted metals of several kinds including iron as a method of producing the ferrous eutectic alloy powder by which the production cost of the ferrous eutectic alloy powder can be effectively reduced. According to the atomization method, the melted metals of several kinds including iron is poured from nozzle and then splashed by a compressed gas or a jet of water jet blown thereto so as to be quenched to solidify, and as a result, the ferrous eutectic alloy powder used as raw material of the ferrous sintered alloy is obtained at reduced production cost.
However, when the atomization method is used for producing the ferrous eutectic alloy powder containing carbon and phosphorus directly from the melted metals of several kinds including iron, the ferrous eutectic alloy powder obtained as a result of splashing and quenching the melted metals is little attended with segregation and is in a stable condition to be homogeneous in its internal structure. Accordingly, when the green compact which is formed through compression molding of the ferrous eutectic alloy powder produced through the atomization method is sintered, liquid phase components which arises as a sequel to melting of low melting point portions of the ferrous eutectic alloy powder forming the green compact are not obtained sufficiently, and therefore the ferrous sintered alloy obtained by sintering the green compact formed by the ferrous eutectic alloy powder produced through the atomization method is provided therein with a large number of pores undesirably so as to have relatively low hardness.
In order to make an improvement in the ferrous sintered alloy obtained from the ferrous eutectic alloy powder produced through the atomization method, it is considered to sinter the green compact formed through compression molding of the ferrous eutectic alloy powder produced through the atomization method at increased sintering temperature so as to increase the liquid phase components arising in the green compact. In such a case, however, there is another problem that several kinds of phosphides each of which is generally hard but brittle are crystallized around each of carbides arising in a matrix structure of the ferrous sintered alloy. Then, when the ferrous sintered alloy in which the phosphides are crystallized is used for making thereof a movable part of an engine having a sliding friction surface, another part of the engine which is in contact with the sliding friction surface of the movable part is caused to have undesirably increased abrasion loss at portions hereof coming into contact with the sliding friction surface of the movable part.